This invention relates generally to radiation systems and methods and in particular to methods of homing and establishing reference frames for motion axes in radiation systems.
Many machines are designed with computer controlled axes or mechanisms which use motors, feedback devices, and sensors to move the axes to precise positions. In cases where an axis includes a relative feedback device which does not have an absolute position throughout the range of the axis motion, the current axis position is not known when power is applied to the system. It is then necessary to move the axis toward a known position and precisely detect this position to establish a home or reference position. Once the reference position is established, the axis can be commanded to move to an accurate location.
In the art a routine for establishing an axis home or reference position is referred to as homing. One conventional homing approach involves the use of home sensors or switches in which an axis is moved toward a home sensor and a reference position is initialized solely based upon triggering this home sensor. This approach typically does not provide good precision since most sensors only trigger with limited accuracy. Further, when only a single home sensor is used, any movement of the sensor such as caused by loose mounting screws will affect the axis positioning.
Another conventional homing approach involves the use of a feedback device and a home switch located at a known location. Either the home switch is used as the reference along with a current position of the feedback device, or once the home switch is triggered, the first index pulse of the feedback device is captured and used as the reference position. An “index pulse” is a single digital signal which a feedback device sends at the exact same rotational angle of the feedback device. In this approach the precision is determined by the combination of the home switch and the index pulse of the feedback device. If either the home switch or the index pulse of the feedback device is moved, the reference location is wrong.
Radiation systems such as medical linear accelerators include various motion axes that operate to produce treatment beams and position patients or other units such as imaging devices. Because of the high radiation environment it would be beneficial to minimize the number of electronic components and simplify the design for robust and reliable homing. For safety of radiation therapy it would be desirable to provide for a homing routine or a checkout that can verify that the correct home position is found in the homing routine to avoid single points of failure. It would be beneficial to establish reference frames for various motion axes of a radiation system that can be automatically calibrated such that the components of the motion axes can be serviced or replaced without the need for retuning of the system after the service or replacement. There is a greater need for reliable homing methods by which high-precision, highly-repeatable, and very durable reference positions can be established for various motion axes in radiation systems.